The present invention relates generally to an adjustable free jet twin-wire former for forming a fiber web. This adjustable twin-wire former has suction boxes on both sides of the twin-wires with at least one of the suction boxes or suction box covers being movable relative to the wires. By controlling the movement of at least one of the suction boxes or the box covers the geometry of the twin-wire may be adjusted based on the drainage characteristics of the liquid-fiber suspension, known as paper stock, in order to improve the overall drainage of the web. Most importantly, the pressure in the forming zone, that is, the pressure exerted on the liquid fiber suspension between the fabrics, is controlled by the pressure exerted by the moveable suction elements on the contacting fabric.
More specifically, the forming geometry of the twin-wire former is adjustable in response to an operator independently controlling the vacuum in the suction boxes and the pressure exerted by the boxes on the forming fabric. This ability to apply vacuum from both sides and to independently control the pressure exerted by the boxes on the forming fabric makes it possible to use much higher drainage forces and a longer forming zone and thus increase the drainage capacity. More importantly, this type of forming zone geometry allows an adjustable twin-wire former to run heavier basis weights, at lower consistencies, and at higher speeds while improving the quality and uniformity of the sheet.
During the last three decades, twin-wire formers have been developed in which the aqueous-fiber suspension is injected between two fabrics or wires in a thin stream or jet. This allows simultaneous drainage of the water through both fabrics and eliminates the limitations caused by free surface flow which limits the speed of a single wire former. Today, these twin-wire formers, often termed free jet gap formers, run the fabrics under tension over a curved surface, made up of either rolls, curved shoes or boxes. Any or all of these curved surface elements can be either drilled or have ribbed surfaces as well as vacuum for additional drainage.
The inside wire of a free jet gap former is traditionally supported by either a rotating or stationary element. The drainage in the inner direction through the inside wire is determined by the pressure exerted on the stock by the outside wire and by the vacuum exerted on the inside wire. If the stationary or rotating element is ribbed or is provided with individual strips in the suction boxes, the drainage pulses due to the pressure variations caused by the ribs or strips which effect a change in cross-sectional area over the ribbed sections of the stationary elements. This pressure change causes shear, which improves the formation of the web and reduces retention and thus affects the fines and filler distribution, as well as affects the wire mark and surface characteristics. By increasing the vacuum level and altering the type of support structure, the properties of the inside wire side of the formed sheet can be varied.
The outside wire exerts a pressure (P) against the stock that is equal to the wire tension (T) divided by the radius of the surface over which it passes (R), eg. ##EQU1## This pressure, exerted by the outside wire, in addition to providing a driving force for drainage, provides stability for the fabric. By both running in an arc and exerting a pressure, the outside wire adjusts its position based on the drainage characteristics of the stock and the drainage along the path. However, because there are no ribs engaging the outside wire, the drainage is under constant pressure and accordingly there is no pulsing on the side of the sheet facing the outside wire. Thus, the two sides of this sheet are formed under different conditions and are not identical, thereby leading to undesirable asymmetrically formed sheets.
In order to provide pulsing on the outside free floating wire, some prior art formers utilize doctors. These stationary elements, blades or strips scrape off the water that drains in this direction providing the necessary shear which improves the formation of the sheet. As mentioned above, this effects the fines and filler distribution, as well as the wire mark and surface characteristics. These line contact devices or doctors are normally used on the outside wire and do not materially affect the position or geometry of the outside wire. The outward drainage force is therefore provided by the pressure exerted by the wire. This pressure for commercially used formers today varies between 0.1 to 1.0 meter water gauge. The drainage in the outward direction is therefore limited and not controllable.
As mentioned above, prior art twin-wire formers with all the stationary elements on one side, will give an asymmetrical sheet. In order to reduce or eliminate this asymmetrical formation, it has previously been suggested that the curvature of the twin-wire former may be changed so that stationary elements can also be put on the outer side as well as on the inside. Such prior art twin-wire formers which are provided with stationary elements on both sides with the elements on one side being moveable (e.g. Duoformer D, Symformer MB) have vacuum applied only to one side and are limited as to drainage capacity as well as require an increased forming zone length.
Another limitation of these prior art twin-wire formers is the fact that the forming pressure for drainage through the outer wire is equal to the pressure between the wires. The pressure between the wires caused by the tension of the outer wire and its radius, in addition to being the driving force for drainage, is the driving force for a lateral sideways movement of the fiber suspension out towards the edges. The distance laterally that the fiber suspension will travel is approximately proportional to the time, t, between the wires times the square root of the pressure, .DELTA.p. When this driving force, the pressure between the wires caused by the tension of the outer wire and its radius, t .sqroot..DELTA.p.sub.' exceeds a certain value, the fiber suspension trapped between the fabrics will get pushed out through the sides of the forming zone. This limits the drainage time and thus the forming length that can be used on gap formers.
As the pressure between the wires is the force for drainage in one direction, (the outer direction) and the vacuum on the other side plus this pressure minus centrifugal force is the drainage force in the other direction (the inner direction), it is possible to effect the sideways movement and its relationship to drainage capacity by increasing the vacuum in a stationary element such as a vacuum box or in a vacuum roll. The importance of the sideways movement can be illustrated easily. When running a twin-wire former, one can show that if the machine is started and run at a certain speed and with water and no fiber, there is hardly any sideways movement and the drainage length is very short. As fibers are added to the water so that the consistency (percentage of fiber) of the stock increases, the drainage length increases, as does the sideways or lateral movement of the stock. At a given point, the aqueous-fiber suspension starts to squirt out from between the wires, sideways. This is the limit of consistency for that particular twin-wire former. Accordingly, the limiting factor for a twin-wire former is basically drainage time, which in turn is dependent on basis weight, drainage resistance, headbox consistency, drainage pressure, temperature and pulsing, depending on what parameters are available for adjustment.
In order to improve the understanding of drainage capacity and the need to increase the drainage capacity of gap formers, a review of the physics of drainage should be considered. The time it takes to drain a fiber suspension under constant pressure without disturbance can be expressed as t=G.times..mu..times.C.sup.-1 .times.W.sup..alpha. .DELTA.p.sup..beta.. Here:
t=drainage time in seconds. PA1 G=drainage resistance constant, characteristic of each furnish. PA1 .mu.=viscosity of water, which drops dramatically with temperature. (The higher the temperature, the lower the viscosity.) PA1 C=consistency of fiber suspension. PA1 W=basis weight in g/m.sup.2 of the paper being formed. PA1 .DELTA.p=pressure drop available for drainage in meters of water (mwg). PA1 .alpha.=exponent that normally is around 2 for relatively free sheets, but can go up to 31/2 for highly beaten stock.
.beta.=-1/2 in most cases.
This equation shows that the drainage curve is basically parabolic against basis weight. That is, drainage time increases at least as the square of the basis weight (assuming .alpha.=2) and is inversely proportional to the headbox consistency and to the square root of the pressure drop, and directly proportional to the viscosity.
The drainage time can be replaced by the active drainage length, L, divided by the speed of the wire, V. The equation can then easily be transformed to a drainage capacity equation by transferring the square root of .sqroot..DELTA.p and V to opposite sides. This gives the equation the form of L.times..sqroot..DELTA.p.sub.' =DC=G.times..mu..times.C.sup.-1 W.sup.2 .times.V. Realizing that the L.times..sqroot..DELTA.p.sub.' is the sum of all the drainage elements in the forming table, one can replace L.times..sqroot..DELTA.p.sub.' with the sum of f.times.L.times..sqroot..DELTA.p.sub.' for each drainage element. This expression corresponds to the drainage capacity of the former if the maximum .DELTA.p is used. The factor f is a constant which is included to take care of the effect of disturbances or pulsing on the drainage resistance of the web.
By pulsing the liquid-fiber suspension, as discussed earlier, some fines are washed out and the liquid-fiber suspension expands thus reducing the drainage resistance. Based on plots of this drainage capacity, as the drainage capacity of each element is additive, it can be shown that the factor f for table rolls is close to 2, for foils about 1.5 and is somewhat less for other stationary elements. Based on this equation, the drainage capacity of a given forming zone can be determined, element by element, by adding the active drainage capacity of each individual element together. This works very well for single wire formers. It can also be applied to twin-wire formers by applying the formula: total drainage capacity, DC.sub.T',=D.sub.1 +D.sub.2 +2.times..sqroot.D.sub.1 .times.D.sub.2.
D.sub.1 and D.sub.2 are the drainage capacities of the inner and outer wire respectively, based on the active drainage elements in each fabric. Using these formulas, the drainage capacity of any single wire or twin-wire former can be calculated with some degree of accuracy. It illustrates that any former has a defined drainage capacity which determines the capability of this former.
Based on the time it takes to drain a fiber suspension under constant pressure, discussed previously, it is obvious that the time to form a web of half weight is only 1/4 of that needed to form a like sized web of full weight. A gap former will then only need one quarter of the drainage time and therefore one quarter of the drainage length (using the same drainage elements) than a single wire forming section producing a web of the same basis weight. This is shown by the drainage capacity equation where, if D.sub.1 +D.sub.2 are equal, the drainage capacity for a gap former with a D.sub.1 drainage capacity on each side is equal to 4D.sub.1. This illustrates why twin-wire forming is such a powerful tool.
The drainage capacity equation can be further changed by including the fact that production per unit width, P, is equal to the product of the basis weight, W, and the speed, v. The drainage capacity, DC, is thus equal to G.times..mu..times.P(production).times.W.times.C.sup.-1. A machine with a given drainage capacity and a fixed production will have to have the headbox consistency raised in proportion to the basis weight to be formed. For a given basis weight and consistency, the required drainage capacity increases in direct proportion to the production rate. However, for the presently available gap formers, the drainage capacity is limited by the sideways flow caused by the product, t.times..sqroot..DELTA.p.sub.f'. This limitation of the drainage capacity of gap formers limits the types of papers that can be made on gap formers.
In the equation t.times..sqroot..DELTA.p.sub.f', which controls the sideways flow, t is equal to the drainage time needed to form the sheet, while .DELTA.pf.sub.f is the pressure exerted in the area between the fabrics. By keeping .DELTA.p.sub.f small, the drainage time, t can be dramatically increased. Also by keeping the .DELTA.p.sub.f low compared to the .DELTA.p.sub.d 's used for drainage through the fabrics, it is possible to greatly increase the drainage capacity of twin wire formers with both .DELTA.p.sub.f and the outward .DELTA.p.sub.d on roll formers being in the order of 1 m w.g. Here, .DELTA.p.sub.f and .DELTA.p.sub.d are basically the same if the centrifugal force is ignored. However, using curved, stationary elements, the .DELTA.p.sub.f and outward .DELTA.p.sub.d are normally in the order of 0.1 m w.g. By using vacuum for controlling drainage in both directions in the order of 0.3 to 3 m w.g., one can increase the drainage capacity on the outside wire by a factor of 1.7 to 10. This allows the formation of sheets with basis weights of up to three times the basis weights that can be formed on conventional twin wire formers.
Further reasons for this limitation of the drainage capacity include the relationship between formation and forming consistency, formation and pulsing, and the relationship between formation and paper properties. Formation, expressed as the standard deviation of the mass of the paper over a very small area, decreases linearly with the inverse of headbox consistency to below 0.1% effective consistency for long fibered pulps with an average fiber length of .about.3 mm. For hardwood pulps with fiber length of .about.1 mm, formation improves linearly with the inverse of the headbox consistency to 0.4 to 0.5%. The formation is considerably better for short fibered stocks than long fibered stocks at the same consistency. For most papers, the effective long fiber consistency determines the formation. There is also a linear relationship between formation and tensile strength as well as many other paper properties such as the printing surface which also improves with formation. At a given consistency, pulsing during the forming process will improve the formation.
Even if the level of formation improves with pulsing, there is still a straight line relationship between formation and effective headbox consistency for each forming section. For a given paper quality, the market determines what is an acceptable formation, which in turn, for a given paper machine and furnish, determines the maximum headbox consistency that can be used to make a satisfactory product. Because of the drainage capacity limitations of gap formers, this has limited the use of gap formers to certain paper grades. Grades that can be successfully made on today's gap formers are tissue, newsprint, light weight coated paper (LWC), SC grades, lighter weight, groundwood specialties, lightweight fine papers and, in special cases, corrugating medium and lightweight liner board. However, the fine papers made on today's gap formers from a substantial amount of long fibers do not have optimum formation, nor do the linerboard sheets. With today's increasing machine speeds and the need for increased paper quality, it is becoming more and more important to develop gap formers with much higher drainage capacity. This is even more important for multi-ply structures. Accordingly, as the drainage capacity needed to form a sheet under a given set of conditions is inversely proportional to the headbox consistency, this means that higher drainage capacity is needed in order to lower the forming consistency and thus improve formation and sheet properties.
As an example, multi-ply fine papers should preferably be made with hardwood fibers on the outside and long softwood fibers on the inside in the center. This gives a sheet with good mechanical properties, such as stiffness, and optimal surface properties for good printing. However, to be able to form a satisfactory multi-layer structure from a furnish with 50% softwood or more, it is necessary to use a twin-wire former with about two to three times the drainage capacity available today. The object of this invention is to make it possible to build twin-wire formers that can accomplish this goal.
According to this invention, this is achieved by controlling the pressure between the wires independently of the forces needed for drainage. This way one can increase the drainage time and the drainage forces without being limited to the same extent by lateral flow as in present day twin-wire forming sections.
The purpose of this invention is to remove the drainage limitations of presently used or available twin wire formers. These twin wire formers have a limited drainage capacity and, therefore, can only be used to make low weight papers and boards. Twin wire formers today, therefore, are used basically for grades like newsprint, LWC, SC papers, mechanical specialties, lighter weight fine papers, corrugating medium and lightweight linerboard. Twin wire formers cannot be used for heavy weight sheets or papers from slow draining stocks. In addition, because of the drainage capacity limitations and in many cases, the papers and boards cannot be formed at consistencies that will give optimum formation. This limited drainage capacity is also the reason that multi-ply papers for grades such as fine papers, linerboard and other board grades have alluded manufacture. Limited drainage capacity also limits the speed at which the different grades can be manufactured. This invention, then, not only improves the quality of the papers and boards and increases the weights where twin wire formers can be used, but also increases the speed at which they can be manufactured.
Because the former uses identical elements and forces on both sides of the sheet, a symmetrical sheet can be produced. This invention also provides action from stationary elements on both sides, which has a very beneficial effect on formation. By using stationary elements and vacuum, control of drainage and fines and filler distribution can easily be achieved. By independently controlling the pressure exerted by the fabrics on the fiber suspension in the wedge and the vacuum exerted by the suction boxes for drainage, it is possible to dramatically increase the drainage capacity, shortening or lengthens the former when advantageous or when it is needed for maximum drainage capacity. This increased drainage capacity, together with the activity on both sides, leads to the ability to make an improved formed sheet with a symmetrical structure.